Optical signal amplifier

ABSTRACT

An optical signal amplifier includes a light source, a depolarizer, and a gain medium that transfers energy from a pump beam output from the depolarizer to the optical signal. The depolarizer may include one or more birefringent optical fibers which support two polarization modes, a fast mode and a slow mode. The light propagates in the fast mode at a higher velocity than the light propagates in the slow mode so as to impart phase delay as the light propagates in the birefringent optical fibers, thereby at least partially depolarizing the beam. A method for using the amplifier with different types of transmission fibers enables the matching of depolarizers with relatively high percentage of degree of polarization, depending on fiber type, while staying below polarization dependent gain requirements.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] The present application is a continuation-in-part of co-pendingand commonly owned U.S. application Ser. No. 09/654,974, filed on Sep.5, 2000, the entire contents of which being incorporated herein byreference.

BACKGROUND

[0002] The present invention relates to fiber optic communicationssystems, and more specifically, to amplification of optical signalspropagating in an optical fiber.

[0003] Optical signals for conveying information in a fiber opticcommunication system experience attenuation as the optical signals aretransmitted though an optical fiber over extended distances. Theattenuated optical signal can be regenerated using amplifiers such asoptical fiber Raman amplifiers, which rely on stimulated Ramanscattering to transfer energy to the optical signal. The optical fiberRaman amplifier comprises a fiber that receives two input beams: a pumpbeam and the optical signal. Energy in the pump beam is coupled into thesignal beam through stimulated Raman scattering, and the optical signalis thereby amplified upon passing through the fiber amplifier. Theextent of amplification or gain depends on the relation between thepolarization of the pump beam and that of the signal beam. If both thepump beam and the signal beam are linearly polarized and have electricfields oriented in the same direction, then the gain is higher than ifthe electric fields are oriented perpendicular to each other.Disadvantageously, fluctuations in the polarization of the signal orpump beam that cause the relative orientations of the electric fields tovary produce fluctuations in the gain of the amplifier. For example, thegain will decrease for pump and signal beams that initially haveelectric fields oriented parallel but are reoriented so as to no longerbe parallel. Conversely, gain will increase if the beams are initiallyperpendicular but subsequently contain parallel components. Suchfluctuations in the gain cause variations in the intensity of theoptical signal, which introduces noise into the signal and therebyincreases the likelihood of errors in transmitting information overoptical fibers.

[0004] In conventional systems designed to minimize fluctuations ingain, the pump beam is provided by two or more semiconductor lasers thatoutput polarized light. The polarized light is directed to a couplerthat combines the light from the different semiconductor lasers afterfirst separating the respective beams into perpendicular polarizations.For example, in the case where two semiconductors are employed to pumpthe fiber amplifier, light emitted from the two semiconductors is inputinto the coupler. The coupler causes the polarized light beams from thetwo semiconductor lasers to have electric fields oriented perpendicularto each other and produces a combined beam that is then directed to theoptical fiber Raman amplifier.

[0005] Although employing a plurality of semiconductor lasers can reducethe fluctuations in gain, requiring more than one semiconductor laseradds to the complexity of the amplifier. What is needed is a design foran optical fiber Raman amplifier that is simpler and less expensive yetthat minimizes the fluctuation in gain caused by variations inpolarization of the pump and signal beams.

SUMMARY

[0006] Methods and apparatus for optical signal amplification areprovided. In one embodiment, an amplifier for amplifying optical signalscomprises a light source having as an output a first beam of lightcharacterized by a first degree of polarization, a depolarizer opticallyconnected to the light source so as to receive the first light beam asan input and having as an output a pump beam characterized by a seconddegree of polarization wherein said second degree of polarization isless than said first degree of polarization. A gain medium is opticallyconnected to the depolarizer so as to receive the optical signal and thepump beam as inputs and is configured to transfer energy from the pumpbeam to the optical signal. The depolarizer advantageously comprises oneor more birefringent optical fibers.

[0007] A method of making an optical signal amplifier in one embodimentof the invention comprises coupling a light source to an input of atleast one birefringent optical fiber and coupling an output of said atleast one birefringent optical fiber to a gain medium.

[0008] Methods of optical signal amplification include collecting lightfrom a light source that emits at least partially polarized lightdivisible into light of two orthogonal linearly polarized states. Thiscollected light is at least partially depolarized by imparting phasedelay between the light of the two orthogonal linearly polarized statesand is then directed into a gain medium of an optical signal amplifier.In another embodiment, a method of minimizing polarization induced gainfluctuations in an optical signal amplifier comprises at least partiallydepolarizing a beam of light from a first light source without combiningthe beam of light with a second beam of light from a second lightsource. This at least partially depolarized beam of light is used as apump beam in the optical signal amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1A is a schematic diagram of an optical communication systememploying an optical amplifier.

[0010]FIG. 1B is a block diagram of a preferred embodiment of theoptical amplifier comprising a pump laser, a depolarizer, and a gainmedium.

[0011] FIGS. 2A-2C are schematic views of preferred embodiments of thepresent invention comprising a non-depolarizing birefringent opticalfiber joined to a depolarizing birefringent optical fiber so as toprovide a mismatch between respective principal axes of the two fibers.

[0012] FIGS. 3A-3C are schematic views of preferred embodiments of thepresent invention comprising a pump laser that emits linearly polarizedlight having an electric field oriented in a fixed direction and adepolarizing birefringent optical fiber having principal axes that arenot aligned with the electric field of the polarized light.

[0013]FIG. 4 is a schematic view of a preferred embodiment of thepresent invention similar to that shown in FIG. 2A additionallycomprising a polarization controller.

[0014]FIG. 5 is a schematic view of a preferred embodiment of thepresent invention comprising a non-depolarizing birefringent opticalfiber coupled to two depolarizing birefringent optical fibers.

[0015]FIG. 6 is a schematic view of a preferred embodiment of thepresent invention similar to that shown in FIG. 5 additionallycomprising a fiber Bragg grating inserted in the non-depolarizingbirefringent optical fiber.

[0016]FIG. 7 is a schematic view of a preferred embodiment of thepresent invention similar to that shown in FIG. 5 additionallycomprising a polarization controller inserted in the non-depolarizingbirefringent optical fiber.

[0017]FIG. 8 is a schematic view of a preferred embodiment of thepresent invention similar to that shown in FIG. 5 additionallycomprising a fiber Bragg grating and a polarization controller insertedin the non-depolarizing birefringent optical fiber.

[0018]FIG. 9A is a schematic view of a preferred embodiment of thepresent invention wherein a plurality of semiconductor lasers andaccompanying depolarizers are coupled to a multi-wavelength opticalcoupler.

[0019]FIG. 9B is a schematic view similar to that shown in FIG. 9A withfiber Bragg gratings inserted between the lasers and depolarizers.

[0020]FIG. 10 is a schematic view of a preferred embodiment of thepresent invention showing the plurality of semiconductor lasers coupledto a plurality of non-depolarizing birefringent optical fibers that arejoined to a plurality of depolarizing birefringent optical fibers thatlead to the multi-wavelength optical coupler.

[0021]FIG. 11A is a schematic view of a preferred embodiment of thepresent invention wherein the plurality of semiconductor lasers arecoupled to the multi-wavelength optical coupler, which is coupled to thedepolarizer.

[0022]FIG. 11B is a schematic view similar to that shown in FIG. 11Awith fiber Bragg gratings inserted between the lasers and themulti-wavelength optical coupler.

[0023]FIG. 12 is a plot, on axes of fiber length, in centimeters (cm),and degree of polarization (DOP), in percent, depicting how the degreeof polarization is reduced with increasing length of the depolarizingbirefringent optical fiber.

[0024]FIG. 13 is a plot, on axes of degree of polarization, in percent,and polarization dependence of gain (PDG), in decibels, illustrating howlowering the degree of polarization reduces the fluctuations in gaincaused by fluctuations in polarization.

[0025]FIG. 14 is a plot, like that shown in FIG. 13, but does notinclude the polarization dependent loss of 0.12 dB of the measuredsystem, which is included in FIG. 13, and the plot shows PDG for avariety of fiber-types.

[0026]FIG. 15 is a histogram of pump source production yield as afunction of % DOP.

[0027]FIG. 16 is an example probability density function of %DOP, and isdivided into different regions, showing which types of systems canaccommodate depolarizers with higher than typical levels of % DOP.

DETAILED DESCRIPTION

[0028] Embodiments of the invention will now be described with referenceto the accompanying Figures, wherein like numerals refer to likeelements throughout. The terminology used in the description presentedherein is not intended to be interpreted in any limited or restrictivemanner, simply because it is being utilized in conjunction with adetailed description of certain specific embodiments of the invention.Furthermore, embodiments of the invention may include several novelfeatures, no single one of which is solely responsible for its desirableattributes or which is essential to practicing the inventions hereindescribed.

[0029] As shown in FIG. 1A, a fiber optical communication system 2comprises a transmitter 4 optically connected to a receiver 6 through anoptical fiber 8. An amplifier 10 such as an optical fiber Ramanamplifier may be inserted between two segments at the optical fiber 8.The transmitter 4 comprises an optical source such as a laser diodewhich emits an optical beam that is modulated to introduce a signal ontothe beam. The optical signal beam is coupled into the optical fiber 8,which carries the beam to the receiver 6. At the receiver 6, the opticalsignal is converted into an electrical signal via an optical detector.To ensure that the optical signal is sufficiently strong such that themodulation can be accurately detected at the receiver 6, amplificationis provided by the optical fiber Raman amplifier 10. Such amplificationis especially critical when the optical signal is transported over longdistances within the optical fiber 8.

[0030] A block diagram of the optical fiber Raman amplifier 10 that is apreferred embodiment of the present invention is shown in FIG. 1B. TheRaman amplifier 10 comprises a light source 12, a depolarizer 14, and again medium 16 and also has an input 18 for the optical signal that isto be amplified and an output 20 for the amplified optical signal. Thelight source 12 may comprise a single light generator or a plurality oflight generators having the same or different wavelengths.

[0031] The light source 12 emits a beam of light represented by a line22 extending from the light source 12 in FIG. 1B. Preferably, the beamof light 22 and the optical signal are separated in wavelength by about50 to 200 nanometers (nm), and more preferably, by about 100 nanometers.The light source 12 may, for example, comprise a semiconductor laser orlaser diode. As is well known in the art, semiconductor laser diodesgenerally emit light that is substantially linearly polarized, i.e.,electromagnetic waves having an electric field oriented in a fixeddirection. To provide a constant level of gain in the gain medium 16, aswill be discussed more fully below, the pump beam preferably comprisessubstantially unpolarized light, not linearly polarized light.Accordingly, the beam 22 is directed to the depolarizer 14, whichreceives the linearly polarized light and at least partially depolarizesthe light. In preferred embodiments, the output of the depolarizer 14comprises at least partially depolarized light. Most preferably, thisoutput comprises substantially unpolarized light; all or substantiallyall of the beam 22 emitted by the light source 12 is depolarized by thedepolarizer 14.

[0032] The light beam 22, after passing through the depolarizer 14 isdirected to the gain medium 16 as depicted by line 24 extending from thedepolarizer to the gain medium. The beam entering the gain medium 24 isreferred to herein as the pump beam. The optical signal is also sent tothe gain medium 16 as illustrated by line 26 in FIG. 1B. The opticalsignal enters the input 18, is amplified within the gain medium 16, andexits the output 20 a stronger signal, which is represented by a line 28emanating from the gain medium. Within the gain medium 16, energy fromthe pump beam 24 is coupled to the signal 26 via stimulated Ramanscattering as is well known in the art.

[0033] As discussed above, the extent of amplification depends on therelation between the polarization states of the pump beam and theoptical signal. The optical signal also comprises electromagnetic waveshaving an electric field and a magnetic field. If the electric field ofthe optical signal is directed parallel to the electric field of thepump beam, the amplification provided by the gain medium 16 will bemaximized. Conversely, if the electric fields are perpendicular to eachother, a minimum in gain results. When the electric fields are not fullyparallel or perpendicular, but contain both parallel and perpendicularcomponents, the gain will have a value somewhere between the minimum andmaximum depending on the magnitude of the parallel and perpendicularcomponents. Accordingly, as the relative orientation of the electricfields in the pump beam and the optical signal vary, the gain will vary.If, however, the pump beam remains entirely unpolarized, containing nopredominant linear polarized component, the gain will not fluctuate.Thus, by passing the light emitted by the light source 12 through thedepolarizer 14, the variations in the amount that the optical signal 26is amplified can be reduced.

[0034] In another configuration, the pump beam itself can be amplifiedby another pump beam using an additional gain medium. In this case,using depolarized light source to pump this additional gain medium willreduce the fluctuation of the power of the pump beam caused bypolarization dependent gain fluctuations.

[0035] FIGS. 2-8 depict preferred embodiments of the optical fiber Ramanamplifier 10 of the present invention in which the depolarizer 14comprises one or more birefringent optical fibers. The one or morebirefringent optical fibers are configured to at least partiallydepolarize light from the light source 12.

[0036] Referring now to FIG. 2A, the light source 12 advantageouslycomprises a semiconductor laser 29 which is coupled through a fiberconnector 30 to a birefringent optical fiber 32. In this embodiment, thebirefringent optical fiber 32 functions as part of the light source 12and does not function as the depolarizer 14 and, therefore, ishereinafter referred to as the non-depolarizing birefringent fiber. Thisnon-depolarizing birefringent optical fiber 32 has a fiber Bragg grating34 inserted therein. The fiber Bragg grating 34 comprises a diffractingreflector, which when employed in association with the semiconductorlaser 29, transmits a wavelength band of light output the laser. Thenon-depolarizing birefringent optical fiber 32 is connected to anotherbirefringent optical fiber 36 that serves as the depolarizer 14 and,accordingly is denoted the depolarizing birefringent optical fiber. Thisdepolarizing birefringent optical fiber 36, along with an input opticalfiber 38 for carrying the optical signal, are attached to an opticalcoupler 40 that leads to the gain medium 16, namely, an optical fiberRaman gain medium, which produces gain through stimulated Ramanscattering. Preferably, the optical fiber Raman gain medium 16 comprisesquartz, and more preferably, ion-doped quartz.

[0037] The non-depolarizing and depolarizing birefringent optical fibers32, 36 are coupled together at a point 42, a close-up of which isdepicted in FIGS. 2B and 2C. As shown in FIGS. 2B and 2C, a longitudinalaxis, z, runs down the length of the non-depolarizing and depolarizingbirefringent optical fibers 32, 36. Mutually perpendicular x(horizontal) and y (vertical) axes extend through and are perpendicularto the z axis.

[0038] The non-depolarizing and depolarizing birefringent optical fiber32, 36 each have a central core and a cladding. As is conventional, thecore has a refractive index that is higher than that of the cladding.Stress imparting layers (not shown) are disposed in the cladding, thecore sandwiched therebetween. As a result of this sandwich structure,the refractive index of the core is different for light linearlypolarized in the x direction and light linearly polarized in theydirection, that is, for electromagnetic radiation having an electricfield parallel to the x axis and electromagnetic radiation having anelectric field parallel to the y axis, respectively. Consequently,linearly polarized light having a polarization parallel to thehorizontal direction travels through the birefringent optical fiber 32,36 at a different velocity than light having a polarization parallel tothe vertical direction. In accordance with convention, and as usedherein, one of these axes, the x axis or the y axis, is referred to asthe fast axis, and the other axis is referred to the slow axis. Lighthaving an electric field aligned with the fast axis, propagates alongthe length of the core at a higher velocity than light having anelectric field aligned with the slow axis. Like the x and y axes, thefast and slow axes are perpendicular. Also as used herein, the termprincipal axes corresponds to the fast and slow axes.

[0039] In this embodiment of the invention, the non-depolarizingbirefringent optical fiber 32 is oriented such that one of the principalaxes of this fiber matches the polarization of the light emitted by thesemiconductor laser 29. For example, the non-depolarizing birefringentoptical fiber 32 may be rotated about its length, the z axis, such thatits fast axis is aligned and parallel with the electric field of theelectromagnetic radiation from the semiconductor laser 29 that istransmitted through the non-depolarizing birefringent fiber.

[0040] Also, in accordance with the present invention, the depolarizingbirefringent optical fiber 36 is oriented such that the principal axesof the non-depolarizing birefringent fiber 32 are not aligned with theprincipal axes of the depolarizing birefringent fiber. An exemplaryarrangement of the non-depolarizing and depolarizing birefringentoptical fibers 32, 36 is shown in FIGS. 2B and 2C where thenon-depolarizing birefringent optical fiber has a principal axis, e.g.,a fast axis, represented by a first arrow 44 while the depolarizingbirefringent optical fiber has a principal axis, also a fast axis,represented by a second arrow 46. The fast axis of the depolarizingoptical fiber 36 is rotated about the length of the fiber, or the zaxis, by a non-zero angle θ with respect to the fast axis of thenon-depolarizing optical fiber 32. As shown in FIG. 2C, the angle θpreferably equals 45°.

[0041] In operation, the semiconductor laser 29 emits a light beamcomprising substantially linearly polarized light that is coupled intothe non-depolarizing birefringent optical fiber 32 by the fiberconnector 30. As discussed above, one of the principal axes, the fast orslow axis, of the non-depolarizing birefringent optical fiber 32 isparallel to the electric field of the pump beam. This arrangementmaintains the polarization of the pump beam as it is transmitted throughthe non-depolarizing birefringent optical fiber 32. The light within thenon-depolarizing birefringent optical fiber 32 passes through the fiberBragg defractive grating 34, which provides a resonator external to thesemiconductor laser 29, thereby stabilizing the wavelength of the pumpbeam and narrowing its bandwidth.

[0042] Also as described above, the principal axes of the depolarizingbirefringent optical fiber 36 are nonparallel to the principal axes ofthe non-depolarizing birefringent optical fiber 32. Accordingly, theelectric field of the pump beam that is transmitted through thenon-depolarizing birefringent optical fiber 32 is nonparallel to boththe fast and slow axes of the depolarizing birefringent optical fiber36. For purposes of understanding, the electric field forelectromagnetic radiation passing through a birefringent fiber can beseparated into two components, one parallel to the fast axis and oneparallel to the slow axes, the vector sum of these two components beingequal to the electric field. Similarly, light comprising the lightsource can be separated into two components, linearly polarized wavespolarized in a direction parallel to the fast axis and linearlypolarized waves polarized parallel to the slow axis. The two sets ofwaves are transmitted through the depolarizing birefringent opticalfiber 36, but at different velocities. Thus, after passing through thedepolarizing birefringent optical fiber 36 and upon reaching the opticalcoupler 40 and the optical fiber Raman gain medium 16, one of the setsof waves, the one polarized parallel to the slow axes, experiences phasedelay with respect to the one polarized parallel to the fast axis.

[0043] The phase delay translates into optical path difference betweenthe two sets of waves. The amount of optical path difference depends onthe disparity in velocity as well as the length of the depolarizingbirefringent optical fiber 36. The longer the optical path difference,the less correlation in phase between the light polarized in a directionparallel to the fast axis and light polarized parallel to the slow axis.For sufficiently long lengths of fiber 36, the optical path differencewill be as much as or longer than the coherence length of the light fromthe semiconductor laser 29, in which case, coherence between the twosets of waves will be lost. No longer being coherent, the relative phasedifference between the two sets of waves will vary rapidly and randomly.

[0044] Unpolarized light can be synthesized from two incoherentorthogonal linearly polarized waves of equal amplitude. Since the lightpolarized in a direction parallel to the fast axis and the lightpolarized parallel to the slow axis are incoherent, orthogonal linearlypolarized light, together they produce unpolarized light. Thisconclusion arises because the two sets of waves, which have orthogonalelectric fields and a relative phase difference that varies rapidly andrandomly, combine to form a wave having an electric field whoseorientation varies randomly. Light with a randomly varying electricfield does not have a fixed polarization. Thus, light having rapidlyvarying polarization states, i.e., unpolarized light, is produced.

[0045] The at least partly depolarized pump beam is directed to theoptical coupler 40, which also receives the optical signal transmittedthrough the input optical fiber 38. The propagation of the opticalsignal through the input optical fiber 38 and to the optical coupler 40is represented by a first arrow 48 shown in FIG. 2A. The two beams, thepump beam and the optical signal, are combined or multiplexed in theoptical coupler 40 and fed into the optical fiber Raman gain medium 16,which transfers energy from the pump beam to the optical signal viastimulated Raman scattering. The optical signal exits the optical fiberRaman gain medium 16 as an amplified signal indicated by a second arrow50 shown in FIG. 2A. Since the pump beam is at least partly depolarizedupon passing through the depolarizing birefringent optical fiber 36, thefluctuations in the amplification provided by the optical fiber Ramangain medium 16 are minimized.

[0046] Another embodiment of the present invention comprises a LYOT typedepolarizer having two birefringent optical fibers, one fiber having alength two times or more as long as the other fiber, i.e., withrespective lengths set by the ratio of 1:2 or 2:1. These two opticalfibers 32, 36 are fused together so that the principal axes thereof areinclined at an angle θ of 45° with respect to each other. The extentthat the depolarizing birefringent optical fiber 36 is rotated about thez axis determines the amount of light that is polarized parallel to thefast axis and the amount of light that is polarized parallel to the slowaxis. When θ equals 45°, as depicted in FIG. 2C, the magnitude of theelectric fields for the waves propagating parallel to the fast and slowaxis are the same; thus, the intensities of the two waves are equal. Asdiscussed above, unpolarized light can be synthesized from twoincoherent orthogonal linearly polarized waves of equal amplitude. Sincethe magnitudes of the two incoherent orthogonal linearly polarized wavesare equivalent, substantially unpolarized light can be produced.

[0047] For other values of θ not equal to 45°, the magnitudes of theelectric fields for the waves propagating parallel to the fast and slowaxis are not the same as for the configuration shown in FIG. 2B. For thepurposes of understanding, the combination of the fast and slow wavescan be separated into a sum of two parts. The first part comprises equalmagnitude orthogonal incoherent waves having electric fields parallel tothe fast and slow axis, the combination of which produces unpolarizedlight. The second part comprises the remainder, a component from thelarger of the two waves, which has an electric field parallel either tothe fast or slow axis. This part is linearly polarized. Thus, a portionof the light will be unpolarized and a portion of the light will belinearly polarized. The pump beam will not be completely depolarized.

[0048] A ratio of the intensities of the polarized component to the sumof the intensities of the polarized and unpolarized components is knownin the art as the degree of polarization (DOP). The DOP is generallyexpressed in percentage. Changing the angle between the principal axesof the non-depolarizing and depolarizing birefringent optical fiber 32,36 changes the DOP. For example, if the angle θ is changed from 45°, oncondition that the depolarizing birefringent optical fiber has the samelength, the degree of polarization (DOP) of the pump beam becomeslarger. Accordingly, the angle between the principal axes of thenon-depolarizing and depolarizing birefringent optical fiber 32, 36, inpart, controls the DOP.

[0049] FIGS. 3A-3C depict other preferred embodiments of the inventionwherein the semiconductor laser 29 is joined to the depolarizingbirefringent fiber 36 through the fiber connector 30. This depolarizingbirefringent fiber 36 is directly attached with the optical coupler 40,which receives the optical input fiber 38 and is connected to theoptical fiber Raman gain medium 16. This depolarizing birefringent fiber36 is also oriented such that its principal axes are not aligned withthe electric field of the beam output by the semiconductor laser 29. Forexample, FIGS. 3B and 3C show light emitted by the semiconductor laser29 that is polarized in the vertical direction as indicated by a firstarrow 52. However, one of the principal axes of the depolarizingbirefringent optical fiber 36 (represented by a second arrow 54) isrotated about the z axis by a non-zero angle θ with respect to thevertical direction. As shown in FIG. 3C, the angle θ preferably equals45° such that equal amounts of light polarized parallel to the fast andslow axes propagate through the depolarizing birefringent optical fiber36.

[0050] In another embodiment of the present invention depicted in FIG.4, the light source 12 additionally comprises a polarization controller56 inserted between the non-depolarizing and depolarizing birefringentoptical fibers 32, 36. Similar to the Raman amplifiers 10 described withreference to FIGS. 2A-2C, the semiconductor laser 29 is coupled to oneend of the non-depolarizing birefringent optical fiber 32 through thefiber connector 30, the non-depolarizing birefringent optical fiberhaving a fiber Bragg grating 34 inserted therein. The other end of thenon-depolarizing birefringent optical fiber 32, however, is joined tothe polarization controller 56, which is connected to the depolarizingbirefringent optical fiber 36. The depolarizing birefringent opticalfiber 36 leads to the optical coupler 40, which is connected to theRaman amplifier gain medium 16. The input optical fiber 38 is alsoattached to the optical coupler 40 as described above.

[0051] The light emitted by the semiconductor laser 29 after passingthrough the non-depolarizing birefringent optical fiber 32 reaches thepolarization controller 56. The polarization controller 56 provides thelight, which is directed into the depolarizing birefringent opticalfiber 36, with a preferred state of polarization. Thus, rather thanrotating the orientation of the depolarizing birefringent optical fiber36 about the z axis, the polarization is rotated about the z axis. Inthe embodiments depicted in FIGS. 2A-2C, as well as those depicted inFIGS. 3A-3C, the depolarizing birefringent optical fiber 36 is rotatedto misalign the principal axis of the depolarizing fiber and theelectric field of the pump beam. In contrast, in the embodiment shown inFIG. 4, the electric field of the light emitted by the laser 29 isrotated with respect to the principal axes of the depolarizingbirefringent optical fiber 36 using the polarization controller 56.

[0052] In either case, the extent of rotation determines the amount oflight polarized parallel to the fast and the slow axes of thedepolarizing birefringent optical fiber 36 or alternatively, the amountof light coupled into fast and slow modes supported by the opticalfiber. The depolarizing birefringent optical fiber 36 supports twoindependent polarization modes, a fast mode and a slow mode; that is,the fiber transmits light polarized parallel to the fast axis and lightpolarized parallel to the slow axis. The linearly polarized pump beamcan be divided into light of two orthogonal linearly polarized states, afirst polarization state corresponding to light coupled into the fastmode and a second polarization state corresponding to light coupled intothe slow mode.

[0053] The amount of light in the first linearly polarized state and thesecond linearly polarized state is determined by the orientation of theelectric field of the light with respect to the fast and slow axis ofthe depolarizing birefringent optical fiber 36. If the light is linearlypolarized in the direction of the fast axis, all the light will becoupled into the fast mode and no light will be coupled into the slowmode. If, however, the light has an electric field directed at an angleof 45° with respect to both the fast and the slow axes, then half thelight will be coupled into the fast mode and half will be coupled intothe slow mode. Similarly, for other linearly polarized states, unequalportions of the light will be coupled into the fast and slow modes ofthe depolarizing birefringent optical fiber. Thus, by varying thepolarization state of the light emitted by the laser 29, and inparticular, by rotating the electric field of linearly polarized laseroutput about the z axis, the portion of the light coupled into the fastand slow modes can be controlled. Preferably, equal portions of thelight are distributed to the fast and slow modes of the depolarizingbirefringent optical fiber. Thus, the polarization controller preferablyis adjusted to provide linearly polarized light having an electric fielddirected at an angle of 45° with respect to both the fast and the slowaxes. With use of the polarization controller 56, the non-depolarizingand depolarizing birefringent optical fibers 32, 36 need not be fixed ina specific orientation about the z axis to achieve this distributionthat optimizes depolarization of the laser light.

[0054] FIGS. 5-8 depict other embodiments of the present invention thatinclude an optical distributor 58 connecting the non-depolarizingbirefringent optical fiber 32 to first and second depolarizingbirefringent optical fibers 36 a, 36 b. As in the Raman amplifiersdescribed above with reference to FIGS. 2-4, the semiconductor laser 29is coupled to the non-depolarizing birefringent optical fiber 32 throughthe fiber connector 30. The non-depolarizing birefringent optical fiber32 leads to the optical distributor 58, which may comprise a wavelengthdivision multiplex (WDM) coupler or a polarization demultiplexer.Preferably, however, the optical distributor 58 preserves thepolarization of the beam passing therethrough. The optical distributor58 is connected to one end of the first and second depolarizingbirefringent optical fibers 36 a, 36 b, which are terminated at anotherend by a beam combiner 60. A single-mode optical fiber 62 extends fromthe beam combiner 60 and leads to the optical coupler 40. As describedabove, the optical coupler 40 receives the input optical fiber 38 and isconnected to the optical fiber Raman gain medium 16.

[0055] In one embodiment, the light beam from the semiconductor laser 29is guided through the non-depolarizing birefringent optical fiber 32 tothe optical distributor 58, which directs equal fractions of the beaminto the first and second depolarizing birefringent optical fibers 36 a,36 b. In this embodiment, the optical distributor 58 directs into thefirst depolarizing birefringent optical fiber primarily only light thatis linearly polarized parallel to the fast axis of the firstdepolarizing birefringent fiber 36 a. Similarly, the optical distributor58 directs into the second depolarizing birefringent optical fiber 36 bprimarily only light that is linearly polarized parallel to the slowaxis of the second depolarizing birefringent fiber. Accordingly, theoptical distributor 58 couples one portion, preferably half, of the beaminto the fast mode of the first depolarizing birefringent optical fiber36 a and another equal portion, preferably the other half, into the slowmode of the second depolarizing birefringent optical fiber 36 b. Thelight in the fast mode propagates at a higher velocity than the lightpropagating the slow mode, thereby imparting phase delay as the lightpropagates in the first and second depolarizing birefringent opticalfibers 36 a, 36 b. As described above, this phase delay translates intooptical path difference. In this embodiment, the first and seconddepolarizing birefringent optical fibers 36 a, 36 b each haveapproximately equal lengths. This length is chosen to produce an opticalpath difference that is sufficiently large to reduce the coherencebetween the two portions (i.e., halves) of the beam and to thereby atleast partially depolarize the beam. Alternatively, the first and seconddepolarizing birefringent optical fibers 36 a, 36b can have differentlengths. In this case, the optical path difference will be caused bothby the disparity in the refractive index and the propagation velocitiesfor the fast and slow polarization modes in the two depolarizingbirefringent optical fibers and by the unequal lengths of the twodepolarizing birefringent optical fibers. Again, the lengths can bechosen such that the optical path difference is sufficient to reduce thecoherence between the two portions (i.e., halves) of the pump beam andto produce a depolarizing effect.

[0056] The two portions of the beam in the first and second depolarizingbirefringent optical fibers 36 a, 36 b, respectively, are combined inthe beam combiner 60. Preferably, the beam combiner 60 comprises apolarization preserving beam combiner and the beams transmitted throughthe first and second birefringent optical fibers 36 a, 36 b are linearlypolarized perpendicular to each other when the pump beam is output fromthe beam combiner.

[0057] In another configuration, the optical distributor 58 directsequal portions of the beam from the laser 29 into the first and secondbirefringent optical fibers 36 a, 36 b without restricting thepolarization of the light. Thus, light is coupled into both the fast andslow modes of the first depolarizing birefringent optical fiber 36 a andinto both the fast and slow modes of the second depolarizingbirefringent optical fiber 36 b. The first and second depolarizingbirefringent optical fibers 36 a, 36 b, however, have different lengths.The difference in length of the two depolarizing birefringent opticalfibers 36 a, 36 b is large enough to produce sufficient optical pathdifference to reduce the coherence between the light in the two fibersand to at least partially depolarize the pump beam. The light in thefirst and second depolarizing birefringent optical fibers 36 a, 36 b iscombined in the beam combiner 60, and this pump beam is directed to theoptical fiber Raman gain medium 16 after being transmitted through thesingle mode optical fiber 62 and coupled with the optical signal in thefiber optic coupler 40.

[0058] Alternatively, equal portions of the beam from the laser 29 arecoupled into the fast mode of the first birefringent optical fiber 36 aas well as the fast mode of the second birefringent optical fiber 36 bor into the slow mode of the first and second birefringent opticalfibers 36 a, 36 b. Additionally, the first and second birefringentfibers 36 a, 36 b have different lengths so as to introduce an opticalpath difference greater than the coherence length between the lightexiting the two fibers. As in the other configurations, the two beamsare brought together in the beam combiner 60, and are directed to theoptical fiber Raman gain medium 16 after being transmitted through thesingle mode optical fiber 62 and combined with the optical signal in thefiber optic coupler 40.

[0059] FIGS. 6-8 differ in that in FIG. 6, the fiber Bragg grating 34 isinserted in the non-depolarizing birefringent optical fiber 32, in FIG.7, the polarization controller 56 is inserted in the non-depolarizingbirefringent optical fiber, and in FIG. 8, both the fiber Bragg gratingand the polarization controller are inserted in the non-depolarizingbirefringent optical fiber. As discussed above, by providing thenon-depolarizing birefringent optical fiber 32 with a fiber Bragggrating 34, an external resonator is formed for the semiconductor laser29. The fiber Bragg grating 34 reflects light from the semiconductorlaser 29 and narrows and stabilizes the wavelength distribution of thelaser output beam. Also as discussed above, the polarization controller56 adjusts the polarization of the beam input to the depolarizer 14 soas to optimize depolarization.

[0060] As shown in FIGS. 9-11, a Raman fiber amplifier 10 may comprise aplurality of semiconductor lasers 29 each emitting a light beam of asame or different wavelength. In one configuration illustrated in FIGS.9A and 9B, a separate depolarizer 14 is associated with each individuallaser 29, with this plurality of depolarizers being optically connectedto a multi-wavelength optical coupler 64. Each of the depolarizers 14receives light emitted from one of the semiconductor lasers 29 andproduces at least partially depolarized light. The resultant pluralityof partly depolarized beams of light are combined into a single pumpbeam within the multi-wavelength optical coupler 64. A separate fiberBragg grating 34 can be inserted between each semiconductor laser 29 andthe respective depolarizer 14 to tailor the wavelength distribution ofthe light output by the semiconductor lasers as shown in FIG. 9B. Thesame methods for producing and depolarizing light beams and foramplifying the signal as described above may be employed for a pluralityof wavelengths. For example, as shown in FIG. 10, each laser 29 in theplurality of semiconductor lasers is coupled to one of the fiberconnectors 30, which is connected to respective non-depolarizingbirefringent optical fibers 32. Each of the non-depolarizingbirefringent optical fibers 32 has the fiber Bragg grating 34 connectedthereto, which is joined to one depolarizing birefringent optical fiber36. Each depolarizing birefringent optical fiber 36 is linked to themulti-wavelength optical coupler 64, which has an optical fiber 66extending therefrom. In general, an optical coupler such as themulti-wavelength optical coupler 64 shown in FIGS. 9A-9B, 10, and11A-11B comprises one or more input lines connected to one or moreoutput lines. The number of input and output lines depends on theapplication. In FIGS. 9A-9B, 10, and 11A-11B, the number of output linesis less than the number of input lines. More specifically, in FIG. 10,three input lines are coupled to the single optical fiber 66. Thisoptical fiber 66 leads to the other optical coupler 40 that receives theinput optical fiber 38. The optical fiber Raman gain medium 16 isattached to this optical coupler 40 as well.

[0061] Each laser 29 emits a beam in a different wavelength band. Thesebeams, which are at least partly depolarized upon passing through theseparate depolarizing birefringent optical fibers 36, are combined inthe multi-wavelength optical coupler 64. The combined beam istransmitted through the optical fiber 66 to the other optical coupler 40and sent on to the optical fiber Raman gain medium 16 along with theoptical signal also received by the optical coupler. In this manner, aplurality of beams having same or different wavelengths can be at leastpartially depolarized and combined to form a pump beam for pumping theoptical fiber Raman gain medium 16. Similarly, in any of the embodimentsdiscussed above, a plurality of semiconductor lasers 29 can be employedto generate a beam comprising light in one or more wavelength bands,which is subsequently depolarized at least partially.

[0062]FIGS. 11A and 11B depict an alternative arrangement wherein themulti-wavelength optical coupler 64 precedes the depolarizer 14. Inparticular, the lasers 29 are connected to non-depolarizing opticalfibers 32 that run to the multi-wavelength optical coupler 64. Asillustrated in FIG. 11B, fiber Bragg gratings 34 can be inserted betweentwo sections of the non-depolarizing optical fibers 32 to control andstabilize the wavelength light emitted by the semiconductor lasers 29.As in the embodiment shown in FIGS. 9A, 9B and 10, the optical fiber 66extends from the multi-wavelength optical coupler 64, however, here theoptical fiber leads to the depolarizer 14.

[0063] Thus, separate light beams having same or different wavelengthsare generated by the plurality of lasers 29. These beams are guidedthrough the non-depolarizing optical fibers 32 and to themulti-wavelength optical coupler 64 where they are combined and outputinto the optical fiber 66. The combined beam travels through the opticalfiber 66 to the depolarizer 14 where the multi-wavelength beam is atleast partially depolarized. After depolarization, the pump beamproceeds to the gain medium 16 as described above. In this manner, alight beam comprising a plurality of same or different laser wavelengthscan be at least partially depolarized and employed to pump the opticalfiber Raman gain 16 medium in the Raman amplifier 10. The use of asingle depolarizer 14 as shown in FIGS. 11A and 11B simplifies the Ramanamplifier 10 as compared to the embodiments depicted in FIGS. 9A, 9B and10, which include a plurality of depolarizers. Depolarization, however,may not be as complete unless the polarization of each of thesemiconductor lasers is aligned, e.g., with individual polarizationtransformers.

[0064] In accordance with the present invention, the length of thedepolarizing birefringent optical fiber 36 can be adjusted to alter thedegree of polarization (DOP). The value of DOP depends on the coherencelength of the pump beam and the optical path difference between thelight coupled into the fast and slow modes of the depolarizingbirefringent optical fiber 36. The optical path difference is determinedin part by the length of the depolarizing birefringent optical fiber 36.Accordingly, DOP depends on the length of the depolarizing birefringentoptical fiber 36. In particular, the polarized component decreases withincreasing length of the depolarizing birefringent optical fiber 36 asshown in FIG. 12, which plots the relationship between the DOP and thelength of the depolarizing birefringent optical fiber. Values for DOPwere measured at the end of the depolarizing birefringent optical fiber36 connected to the optical coupler 40. This plot confirms that the DOPcan be controlled by adjusting the length of the depolarizingbirefringent optical fiber or fibers. It will be appreciated that anydecrease in the polarization of the beam prior to entering the gainmedium is advantageous. However, using the depolarization principles ofthe present invention, the degree of polarization (DOP) of the pump beamis advantageously decreased to at least about 40% or less. Morepreferably, the DOP is decreased below approximately 20%. It has beenfound that the DOP of the pump beam can be reduced to less than about10% in some embodiments of the invention.

[0065] As described above, varying the DOP of the pump beam can controlfluctuations in the optical fiber Raman gain. The level of fluctuationsin gain is characterized by the polarization dependence of the opticalfiber Raman gain (PDG), which is determined by measuring the differencebetween the maximum and minimum value of gain while changing the stateof polarization of the signal being amplified. Measurements of PDGquantifies polarization dependent loss of the optical amplifier 10. FIG.13 plots the PDG as the DOP of the pump beam is reduced using apreferred embodiment described above. The plot shows that the PDGdecreases as the degree of polarization decreases, the PDG becomingcloser to a value of polarization dependent loss, which in this case isequals 0.12 dB (which is primarily PDL attributable to the measuredsystem). Thus, optical pumping of an optical fiber Raman gain medium 16with laser light that has been at least partially depolarized lightreduces the fluctuations in the optical fiber Raman gain.

[0066] Accordingly, employing the depolarizer 14 in the fiber opticalRaman amplifier 10 enables the polarization dependent gain fluctuationsto be reduced. Stable gain is possible while using a singlesemiconductor laser 29 to pump the optical fiber Raman gain medium 16.The laser output need not be combined with light from a second source.The complexity of the Raman amplifier 10 is thus reduced as lesssemiconductor laser devices are required to optically pump the opticalfiber Raman gain medium 16. As illustrated in FIGS. 2-11, this Ramanamplifier 10 can operate with or without the inclusion of the fiberBragg grating 34. However, optical pumping with light having a narrowwavelength distribution is advantageously provided by employing thefiber Bragg grating 34.

[0067] Although a plurality of Raman amplifiers 10 having differentschemes for depolarizing the pump beam are shown in FIGS. 2-11, otherdepolarizers 14, such as other LYOT type depolarizers as well as Cornutype depolarizers can be employed in accordance with the invention toproduce an at least partly depolarized pump beam. Accordingly, thedepolarizer 14 may comprise birefringent components other thanbirefringent fiber such as birefringent crystal. Nevertheless, fiberdepolarizers like the LYOT fiber depolarizer are preferred forintegration into a fiber optic communication system 2. Additionally,other components within the optical amplifier 10 may comprise opticalfiber, optical integrated waveguide devices, or both. For example, anyof the optical couplers (optical coupler 40, optical distributor 58,beam combiner 60, multi-wavelength optical coupler 64) may be fiber orintegrated optic waveguide devices or combinations thereof.

[0068] Furthermore, as described above, the semiconductor laser lightsources 29 output substantially linearly polarized light, which can beat least partially depolarized so as to avoid variation in gain provideby the amplifier 10. The usefulness of the depolarizer 14, however, isnot so limited, that is, the methods describe herein can be employed forlight sources that output non-linearly polarized light. For example,circularly or elliptically polarized light can be at least partiallydepolarized, e.g., by coupling this light into a birefringent opticalfiber, so as to minimize fluctuations in amplification provided by theRaman gain medium 16.

[0069]FIG. 14 is a graph like that of FIG. 13, althoughpolarization-dependent-loss in the measurement system has been removedso that the PDG curves intersect the origin of the graph. Furthermore,data is plotted for several different types of optical fibers. The DCFfiber has the lowest PDG as %DOP increases from 0 to 100. The data alsoincludes non-zero dispersion shifted optical fibers for both forward andbackward pumping. The results indicate that the backward pumpedNZ-dispersion shifted fiber exhibited between 0 dB and slightly lessthan 0.9 dB of PDG over a range of 0 to 100% DOP. The forward pumpedNZ-dispersion shifted fiber exhibited a much greater sensitivity to PDG,experiencing about 1 dB of PDG for a DOP of slightly less than 30%. FIG.14 also includes data for SMFs for both forward and backward propagationmodes, with the SMF with the backward pumping scheme experiencing 1 dBof PDG for about 75% DOP, and the SMF with the forward pumping schemeexperiencing 1 dB of PDG for slightly over 30% DOP.

[0070] Each of the graphs exhibit some linear characteristics (on thePDG in dB vs. % DOP scale) for larger amounts of PDG, thus allowing fora prediction of the amount of PDG (dB) that will be experienced for agiven percentage of % DOP for a given type of fiber. For example, ifonly 0.15 dB of PDG is permissible in a system, and that system used aforward pumped DCF, then the length of the depolarizer fiber could beset to provide 40% or less to meet the system characteristics. Likewise,the graph of FIG. 14 shows that (1) the DOP for NZ backward pumped fiberwould require about 20% or less DOP, (2) the SMF backward pumped fiberwould require 15% or less DOP, while each of SMF forward and NZ forwardpumped fibers would require less than 10%. For larger levels of PDG, dueto the largely linear relationships, if the amount of permissible PDG ischanged (to say 0.2 dB), the tolerable about of % DOP that yields thatamount (or less) PDG can readily be determined from FIG. 14, based onthe type of fiber that is used.

[0071]FIG. 15 is a histogram, with an overlay of a cumulativedistribution function, of an exemplary production yield vs. % DOP thatcould be expected from a PMF (e.g., PANDA-based fiber) set to be 20 mand having a center wavelength of 1476 nm and 1496 nm. The beat lengthof the PMF was set to around 4.5 mm, but not more than 5 mm. Theamplifier fiber was presumed to be a SMF, but other fibers could havebeen used as well. The 20 m PMF length, in an ideal situation, shouldhave produced less than 0.01% DOP and close to 0 dB PDG, although atarget DOP was less than 10% was deemed to be acceptable. Under theseconditions,

[0072] 4 out of 80 samples achieved between 1% and 2% DOP;

[0073] 6 out of 80 samples achieved between 2% and 3% DOP;

[0074] 10 out of 80 samples achieved between 3% and 4% DOP;

[0075] 9 out of 80 samples achieved between 4% and 5% DOP;

[0076] 6 out of 80 samples achieved between 5% and 6% DOP;

[0077] 9 out of 80 samples achieved between 6% and 7% DOP;

[0078] 4 out of 80 samples achieved between 7% and 8% DOP;

[0079] 4 out of 80 samples achieved between 8% and 9% DOP;

[0080] 5 out of 80 samples achieved between 9% and 10% DOP;

[0081] 2 out of 80 samples achieved between 10% and 11% DOP;

[0082] 4 out of 80 samples achieved between 11% and 12% DOP;

[0083] 3 out of 80 samples achieved between 12% and 13% DOP;

[0084] 9 out of 80 samples achieved between 13% and 14% DOP;

[0085] 0 out of 80 samples achieved between 14% and 15% DOP;

[0086] 2 out of 80 samples achieved between 15% and 16% DOP;

[0087] 1 out of 80 samples achieved between 16% and 17% DOP;

[0088] 0 out of 80 samples achieved between 17% and 18% DOP;

[0089] 1 out of 80 samples achieved between 18% and 19% DOP;

[0090] 0 out of 80 samples achieved between 19% and 20% DOP;

[0091] 1 out of 80 samples achieved between 20% and 21% DOP;

[0092] Thus, the actual results show that the production samplesfollowed a random process with regarding to % DOP. In the example ofFIG. 15, the production distribution has a statistical mean that fallssomewhere between 5% and 9%, and a mode at a lesser value (3% in thecase of FIG. 15). Due to this random nature, it is certainly possible tomanufacture devices with less than 1% DOP, although the manufacturingefficiency would be quite low since the production yield would be small.

[0093] Contrary to conventional design practice, the present inventorsrecognized that it is not always necessary to strive for such low DOP aslong as the effect of the higher DOP does not give rise to a greaterthan specified amount of PDG, for a given fiber type. Moreover, byidentifying the relationships shown in FIG. 14 for the different fibertypes, the present inventors recognized that they could make use ofdepolarizers that exhibited higher than conventionally specified amountsof %DOP. For example, a depolarizer that exhibited a 5% DOP would havebeen determined to be unacceptable in conventional devices whose designspecification required 1% or less DOP. Although, according to theobservations of the present inventors, the same depolarizer couldoperate successfully if used in a system that could tolerate less than0.2 dB of PDG and used either DCF or NZ forward pumped fibers, as wellas other fiber-based systems, as is evident from FIG. 14. Accordingly,the identification by the inventors of the relationships shown in FIG.14 enabled much more efficient manufacturing processes, because devicesthat would other have been discarded according to conventional designpractices, were determined to be suitable for use in certain systemswith predetermined PDG requirements and types of transmission (oramplifier medium) fibers.

[0094]FIG. 16 is an exemplary probability density function (pdf) curvethat is divided into five different regions. Not unlike the histogram ofFIG. 15, FIG. 16 is an idealistic pdf of a production yield, fordepolarizer devices that exhibit different percentages of DOP, whichwere designed for producing a low DOP, e.g., less than 1%. As seen inFIG. 16, the X axis is % DOP for devices that are made in series ofproduction runs. The Y axis is the probability of having a particulardevice exhibit a certain percentage of DOP, even though designed toprovide less than 1% DOP. The actual shape of the PDF will depend on avariety of production-based factors relating to how the particulardevices are produced as well as the initial design criteria.

[0095] The pdf is divided into five different regions. Region 1 is theregion that covers depolarizers that exhibit the largest % DOP, above20%, for a particular design criteria, such as 0.15 dB PDG, and fordifferent types of fibers, like those shown in FIG. 14. The regions showwhich systems, each having a different transmission fiber, would besuitable for use with the depolarizers whose % DOP is much greater thanthat of the initial design goal. As an example, Region 1 shows thatdevices falling in this region are suitable for use in a DCF forwardpumped system in which the PDG design criteria was 0.15 dB. However, adevice that is produced with a % DOP falling between 15% and 20% (Region2), could be used in either a DCF forward pumped system or an NZdispersion shifted fiber based system that is backward pumped (see FIG.14). Likewise, Region 3 includes devices having a % DOP between 10% and15% that would be suitable for use in a DCF forward pump system, NZdispersion shifted fiber system that is backward pumped, or a singlemode fiber based system that is backward pumped. Region 4, whichincludes devices having a percent DOP between about 7% and 10%, would besuitable for use for all systems like that described in Regions 1, 2 or3, but also would be suitable for use in a SMF forward pumped system.Region 5, namely devices between 0% DOP and 7% DOP would be suitable foruse in any of the transmission systems, including a NZ dispersionshifted fiber based system that is forward pumped.

[0096] Accordingly, by understanding the performance of differentsystems having depolarizers that exhibit certain percentages of DOP, asshown in FIG. 14, will provides an insight into levels of resulting PDGfor different types of fiber based systems. FIG. 16 provides guidancefor which systems could acceptably accommodate depolarizers, which wouldotherwise be discarded in conventional systems having a design criteriaof less than 1% DOP.

[0097] The present invention may be embodied in other specific formswithout departing from the essential characteristics as describedherein. The embodiments described above are to be considered in allrespects as illustrative only and not restrictive in any manner. As isalso stated above, it should be noted that the use of particularterminology when describing certain features or aspects of the inventionshould not be taken to imply that the terminology is being re-definedherein to be restricted to including any specific characteristics of thefeatures or aspects of the invention with which that terminology isassociated. The scope of any invention is, therefore, indicated by thefollowing claims rather than the foregoing description. Any and allchanges which come within the meaning and range of equivalency of theclaims are to be considered in their scope.

What is claimed is:
 1. An optical signal amplifier comprising: at leastone source of pumping light, said source being configured to producepumping light having a predominant polarization state; at least onedepolarizer comprising a birefringent optical component having aprincipal axis oriented at about 45 degrees with respect to saidpredominant polarization state and coupled to receive said pumping lightas an input and having as an output a pumping beam, wherein said outputpumping beam has a degree of polarization in an inclusive range ofgreater than 1% through approximately 40%; and a Raman gain mediumconfigured to receive said pumping beam and optical signals as inputsand to transfer energy from said pumping beam to said optical signalsvia stimulated Raman scattering.
 2. The amplifier of claim 1, whereinsaid Raman gain medium being a single mode optical fiber.
 3. Theamplifier of claim 2, wherein said Raman gain medium being a single modefiber that is forward pumped.
 4. The amplifier of claim 2, wherein saidRaman gain medium being a single mode fiber that is backward pumped. 5.The amplifier of claim 4, wherein said depolarizer is configured toprovide an output pumping beam that has a degree of polarization in aninclusive range of greater than 1% through 15%.
 6. The amplifier ofclaim 5, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of2% through 13%.
 7. The amplifier of claim 6, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of 3% through 11%.
 8. The amplifierof claim 7, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of4% through 10%.
 9. The amplifier of claim 8, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of 5% through 9%.
 10. The amplifierof claim 9, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of6% through 7%.
 11. The amplifier of claim 3, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of above 1% through 10%.
 12. Theamplifier of claim 11, wherein said depolarizer is configured to providean output pumping beam that has a degree of polarization in an inclusiverange of above 2% through 9%.
 13. The amplifier of claim 12, whereinsaid depolarizer is configured to provide an output pumping beam thathas a degree of polarization in an inclusive range of above 3% through7%.
 14. The amplifier of claim 13, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of above 4% through 6%.
 15. Theamplifier of claim 1, wherein said Raman gain medium being a non-zerodispersion shifted fiber.
 16. The amplifier of claim 15, wherein saidRaman gain medium being a non-zero dispersion shifted fiber that isbackward pumped.
 17. The amplifier of claim 16, wherein said depolarizeris configured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of greater than 1% through 20%. 18.The amplifier of claim 17, wherein said depolarizer is configured toprovide an output pumping beam that has a degree of polarization in aninclusive range of 2% through 16%.
 19. The amplifier of claim 18,wherein said depolarizer is configured to provide an output pumping beamthat has a degree of polarization in an inclusive range of 3% through12%.
 20. The amplifier of claim 19, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of 4% through 8%.
 21. The amplifierof claim 20, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of5% through 7%.
 22. The amplifier of claim 21, wherein said depolarizeris configured to provide an output pumping beam that has a degree ofpolarization of about 6%.
 23. The amplifier of claim 15, wherein saidRaman gain medium being a non-zero dispersion shifted fiber that isforward pumped.
 24. The amplifier of claim 23, wherein said depolarizeris configured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of above 1% through 9%.
 25. Theamplifier of claim 24, wherein said depolarizer is configured to providean output pumping beam that has a degree of polarization in an inclusiverange of above 2% through 8%.
 26. The amplifier of claim 25, whereinsaid depolarizer is configured to provide an output pumping beam thathas a degree of polarization in an inclusive range of above 3% through7%.
 27. The amplifier of claim 26, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of above 4% through 6%.
 28. Theamplifier of claim 27, wherein said depolarizer is configured to providean output pumping beam that has a degree of polarization in an inclusiverange of about 5%.
 29. The amplifier of claim 1, wherein said Raman gainmedium being a dispersion compensation fiber that is forward pumped. 30.The amplifier of claim 29, wherein said depolarizer is configured toprovide an output pumping beam that has a degree of polarization in aninclusive range of greater than 1% through 20%.
 31. The amplifier ofclaim 30, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of2% through 16%.
 32. The amplifier of claim 31, wherein said depolarizeris configured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of 3% through 12%.
 33. The amplifierof claim 32, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of4% through 8%.
 34. The amplifier of claim 33, wherein said depolarizeris configured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of 5% through 7%.
 35. The amplifierof claim 34, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization of about 6%.
 36. A methodof managing polarization dependent gain in a Raman amplifier comprising:routing laser light having a predominant polarization state through asingle birefringent component that has a principal axis oriented atabout 45 degrees with respect to said predominant polarization state soas to produce a pumping beam which has a degree of polarization in aninclusive range of greater than 1% through about 40%; and routing saidpumping beam to a Raman gain medium.
 37. The method of claim 36, whereinsaid Raman gain medium being a single mode optical fiber.
 38. The methodof claim 37, wherein said Raman gain medium being a single mode fiberthat is forward pumped.
 39. The method of claim 37, wherein said Ramangain medium being a single mode fiber that is backward pumped.
 40. Themethod of claim 39, wherein said depolarizer is configured to provide anoutput pumping beam that has a degree of polarization in an inclusiverange of greater than 1% through 15%.
 41. The method of claim 40,wherein said depolarizer is configured to provide an output pumping beamthat has a degree of polarization in an inclusive range of 2% through13%.
 42. The method of claim 41, wherein said depolarizer is configuredto provide an output pumping beam that has a degree of polarization inan inclusive range of 3% through 11%.
 43. The method of claim 42,wherein said depolarizer is configured to provide an output pumping beamthat has a degree of polarization in an inclusive range of 4% through10%.
 44. The method of claim 43, wherein said depolarizer is configuredto provide an output pumping beam that has a degree of polarization inan inclusive range of 5% through 9%.
 45. The method of claim 44, whereinsaid depolarizer is configured to provide an output pumping beam thathas a degree of polarization in an inclusive range of 6% through 7%. 46.The method of claim 38, wherein said depolarizer is configured toprovide an output pumping beam that has a degree of polarization in aninclusive range of above 1% through 10%.
 47. The method of claim 46,wherein said depolarizer is configured to provide an output pumping beamthat has a degree of polarization in an inclusive range of above 2%through 9%
 48. The method of claim 47, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of above 3% through 7%.
 49. Themethod of claim 48, wherein said depolarizer is configured to provide anoutput pumping beam that has a degree of polarization in an inclusiverange of above 4% through 6%.
 50. The method of claim 36, wherein saidRaman gain medium being a non-zero dispersion shifted fiber.
 51. Themethod of claim 50, wherein said Raman gain medium being a non-zerodispersion shifted fiber that is backward pumped.
 52. The method ofclaim 51, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range ofgreater than 1% through 20%.
 53. The method of claim 52, wherein saiddepolarizer is configured to provide an output pumping beam that has adegree of polarization in an inclusive range of 2% through 16%.
 54. Themethod of claim 53, wherein said depolarizer is configured to provide anoutput pumping beam that has a degree of polarization in an inclusiverange of 3% through 12%.
 55. The method of claim 54, wherein saiddepolarizer is configured to provide an output pumping beam that has adegree of polarization in an inclusive range of 4% through 8%.
 56. Themethod of claim 55, wherein said depolarizer is configured to provide anoutput pumping beam that has a degree of polarization in an inclusiverange of 5% through 7%.
 57. The method of claim 54, wherein saiddepolarizer is configured to provide an output pumping beam that has adegree of polarization of about 6%.
 58. The method of claim 50, whereinsaid Raman gain medium being a non-zero dispersion shifted fiber that isforward pumped.
 59. The method of claim 58, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of above 1% through 9%.
 60. Themethod of claim 59, wherein said depolarizer is configured to provide anoutput pumping beam that has a degree of polarization in an inclusiverange of above 2% through 8%.
 61. The method of claim 60, wherein saiddepolarizer is configured to provide an output pumping beam that has adegree of polarization in an inclusive range of above 3% through 7%. 62.The method of claim 61, wherein said depolarizer is configured toprovide an output pumping beam that has a degree of polarization in aninclusive range of above 4% through 6%.
 63. The method of claim 62,wherein said depolarizer is configured to provide an output pumping beamthat has a degree of polarization in an inclusive range of about 5%. 64.The method of claim 36, wherein said Raman gain medium being a forwardpumped dispersion compensating fiber.
 65. The method of claim 64,wherein said depolarizer is configured to provide an output pumping beamthat has a degree of polarization in an inclusive range of greater than1% through 20%.
 66. The method of claim 65, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of 2% through 16%.
 67. The method ofclaim 66, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of3% through 12%.
 68. The method of claim 67, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization in an inclusive range of 4% through 8%.
 69. The method ofclaim 68, wherein said depolarizer is configured to provide an outputpumping beam that has a degree of polarization in an inclusive range of5% through 7%.
 70. The method of claim 69, wherein said depolarizer isconfigured to provide an output pumping beam that has a degree ofpolarization of about 6%.
 71. (Amended) A light source for pumping aRaman gain medium in a Raman amplifier comprising: a laser light sourceconfigured to produce an output light beam having a predominantpolarization state; a single birefringent component having an input portand an output port, wherein said input port is configured to receivesaid output light beam and having a principal axis oriented at about 45degrees with respect to said predominant polarization state so as toproduce a pumping beam which has a degree of polarization in aninclusive range of greater than 1% through about 40%, and wherein saidoutput port is configured to couple said pumping beam to said Raman gainmedium.
 72. The light source of claim 71, wherein said Raman gain mediumbeing a single mode optical fiber.
 73. The light source of claim 72,wherein said Raman gain medium being a single mode fiber that is forwardpumped.
 74. The light source of claim 73, wherein said Raman gain mediumbeing a single mode fiber that is backward pumped.
 75. The light sourceof claim 71, wherein said Raman gain medium being a non-zero dispersionshifted optical fiber.
 76. The light source of claim 75, wherein saidRaman gain medium being a non-zero dispersion shifted optical fiber thatis forward pumped.
 77. The light source of claim 75, wherein said Ramangain medium being a non-zero dispersion shifted fiber that is backwardpumped.
 78. The light source of claim 71, wherein said Raman gain mediumbeing a dispersion compensating fiber that is forward pumped.